The invention concerns a double nut for the controlled fixing (tightening or securing) of a part or component by means of a bolted connection. The double nut comprises a pulling sleeve with an internal thread, which can be screwed onto the threaded end of the bolt, and with an external thread onto which a pressing sleeve, which is supported on the part or component that is to be fixed by means of the bolt connection, can be screwed. The external thread of the pulling sleeve also serves to screw on a pull bushing of a hydraulic bolt-tightening cylinder, which comprises a support bushing that engages over the pulling sleeve and the pressing sleeve and is also supported on the component that is to be fixed by means of the bolted connection.
A double nut of the aforementioned kind and a hydraulic bolt-tightening cylinder that serves for tightening or fixation are described in U.S. Pat. No. 4,844,418 A. The pulling sleeve comprises an external thread that extends over the entire length of the pulling sleeve. By means of its internal thread, the pressing sleeve is screwed onto the external thread of the pulling sleeve. Likewise, the internal thread of the pressing sleeve extends over the entire length of that sleeve, and a collar or flange integrally formed with the pressing sleeve reaches underneath the pulling sleeve.
In order to tighten the bolt connection, the pull bushing of a hydraulic bolt-tightening cylinder is screwed onto the free end of the pulling sleeve and the bolt-tightening cylinder is pressurized. The support bushing is supported on a support surface of the component that is to be fixed and expands or stretches the bolt so that the pressing sleeve is lifted upwardly and away from the support surface. Subsequently, the pressing sleeve can be screwed down towards the support surface, and the collar on the pressing sleeve moves away from the corresponding end of the pulling sleeve. When the pressure in the bolt-tightening cylinder is relieved, the tension in the bolt is conveyed through the pulling sleeve, the pressing sleeve and the collar on the pressing sleeve to the support surface of the component that is to be fixed, so that the initial tension in the bolted connection is retained.
In the known embodiment of the double nut the stress on the lower threads of the pulling sleeve is significantly higher than the stress on those threads that are in the area where the thread of the pressing sleeve ends. That limits the initial tension that can be applied to the bolt connection, especially if, due to space reasons, the walls of the pulling sleeve and the pressing sleeve have to be kept thin compared to the diameter of the bolt.
It is therefore an object of the invention to configure the double nut in such a way that the distribution of stress on the threads between the pulling sleeve and the pressing sleeve is as uniform as possible in order to achieve as efficient a utilization of material as possible while the walls of the pulling sleeve and the pressing sleeve are thin.